Capturing Solar Images in the Extreme Environment of Space

MARIE FREEBODY, CONTRIBUTING EDITOR

Imagine trying to take pictures or make optical measurements on a cloudless day beneath the blistering onslaught of 13 suns. This is the equivalent reality for the European Space Agency’s (ESA’s) Solar Orbiter and NASA’s Parker Solar Probe (PSP), the latter of which is currently operating in temperatures of almost 1640 K in the inhospitable environment around the sun.

The Extreme Ultraviolet Imager (EUI) on ESA’s Solar Orbiter spacecraft captured full-disk images of the sun on May 30, when the spacecraft navigated closer to the star than any previous telescope. Its images show the sun’s appearance at a wavelength of 17 nm, which is in the extreme-UV (EUV) region of the electromagnetic spectrum. Images at this wavelength reveal the upper atmosphere of the sun, the corona, which has a temperature in excess of of 1 million kelvin. Courtesy of ESA and NASA.

“Probably the single important aspect of both of these missions is that they are going to regions of space that have never been explored and are already giving new observations that are changing or explaining our previous concepts of how the sun works,” said Russell A. Howard, head of the Flight Projects Section at the Solar and Heliospheric Physics Branch of the U.S. Naval Research Laboratory (NRL) in Washington, D.C.

The challenge for teams at ESA and NASA was to design these spacecraft systems to operate without melting or warping in such intense heat — not to mention the need to protect the systems from the barrage of dust particles that orbit the sun and the plasma particles that make up the solar wind. The teams’ solutions pull together several innovative technologies from previous missions and draw upon advancements from other disciplines to develop new materials, coatings, and processes.

Our lives explicitly depend on the sun, not just for light and heat, but also to support agriculture and even our increasingly high-tech energy infrastructure. But the sun’s beneficence often comes at a cost.

“The sun is the most powerful particle accelerator in the solar system. It regularly emits ‘storms’ of particles at close to the speed of light. These can penetrate the protective layers of Earth’s magnetic field and atmosphere and their effects can even be detected at the surface of our planet,” said Daniel Müller, Solar Orbiter project scientist at ESA. “Solar energetic particle events are an extreme form of space weather and can severely affect space hardware. They can disrupt radio communications and cause commercial air traffic to be routed away from polar regions — where the energetic particles find it easy to penetrate our atmosphere.”

First-light data from the Parker Solar Probe’s (PSP’s) Wide-field Imager for Solar PRobe (WISPR) instrument suite. The image from WISPR’s outer telescope has a 58° field of view and extends to about 160° from the sun (left). The image from the inner telescope has a 40° field of view, with its right edge 58.5° from the sun’s center. The bright object slightly to the right of center is Jupiter (right). There is a parallax of about 13° in the apparent position of the sun as viewed from Earth and the PSP. Courtesy of NASA/Johns Hopkins APL/Naval Research Lab.

Although the sun is close to Earth compared with other stars, unravelling its secrets even from this distance — defined as one astronomical unit (AU) or ~93 million miles — has proven challenging. For instance, we still don’t know how the solar corona — the sun’s hot outermost atmosphere — maintains temperatures in excess of 1 million kelvin, whereas the visible surface has temperatures of just below 6000 K.

It is the corona that produces the solar wind, an outflow of plasma particles made of ions and free electrons. This outflow bathes our magnetosphere in a continual stream of particles that causes the much-admired aurora, as well as a host of less desirable effects such as communication interference, GPS errors, and satellite drag.

Perhaps the most significant issue is the impact of huge eruptions of material from the sun, known as coronal mass ejections, that cause massive disruptions, including power blackouts. Predicting these eruptions would allow scientists to take steps to mitigate the severity of their influence.

Groundbreaking space missions

ESA’s spacecraft will bring optical cameras and in situ instruments to within 42 million kilometers (26 million miles) from the sun. The craft’s novel design dissipates heat while preserving the rest of the spacecraft within a protective cone of shadow, and its cameras peek through a shield that took years to conceive.

The Solar Orbiter’s Polarimetric and Helioseismic Imager (PHI) instrument consists of three complementary telescopes. Two are high-resolution telescopes that leverage two-mirror configurations to image a fraction of the solar disk at resolutions reaching almost 200 km at perihelion (the minimum orbital distance to the sun). The third telescope, called the Full Sun Imager, relies on a single mirror to image the full solar disk at all phases of the orbit.

All three instruments gather extreme-UV (EUV) photons on CMOS active pixel sensors with formats of 3072 × 3072 pixels measuring 10 μm each. Sensors for the high-resolution-image channels are windowed to 2048 × 2048 illuminated pixels. Developed under the leadership of professor Sami Solanki of the Max Planck Institute for Solar System Research in Germany, the PHI uses special heat- rejecting entrance windows to reflect away most wavelengths from 200 nm to the far-infrared.

The HRI_LYA telescope, part of the EUI instrument on the Solar Orbiter, captures high-resolution images of the solar surface at the so-called Lyman-alpha wavelength (121.6 nm), which is produced by hydrogen, the most abundant chemical element in the universe. The violet color has been artificially added to help with visual identification. Courtesy of ESA and NASA.

“These are multilayer filters having more than 80% transmittance in a narrow notch around the wavelength of scientific interest (617 nm),” said César García Marirrodriga, project manager of ESA’s Solar Orbiter. “The small fraction of unwanted energy that is absorbed by the filter is mostly emitted back to cold space, thanks to a low IR-emissivity coating on the detector side of the filter.”

NASA’s PSP ventures even closer to the sun. But rather than capture direct images of the sun’s surface, the probe aims to identify the mechanisms that heat the corona and accelerate the solar wind. Its only onboard imager takes pictures while looking toward the side of the sun.

To do this, the PSP carries four onboard instruments to capture in situ measurements of the sun’s particles and fields. The first, the Electromagnetic Fields Investigation (FIELDS) instrument, employs magnetometers to help scientists understand why the solar corona is so hot compared to the surface of the sun and why the solar wind moves so quickly. Second, the Solar Wind Electrons, Alphas, and Protons (SWEAP) instrument incorporates four sensors designed to measure characteristics of the solar wind, including electrons, ions of hydrogen, and helium. Third, the Integrated Science Investigation of the Sun Energetic Particle Instruments (IS?IS) also measures energetic particles originating from the sun, including electrons, protons, and ions.

The fourth instrument, the Wide-field Imager for Solar PRobe (WISPR), is the PSP’s imaging component. The white light images captured by this heliospheric imager provide large-scale context of the solar wind, shocks, solar ejecta, and other structures monitored by the craft’s three other in situ instruments. WISPR encompasses a 95° radial by 58° transverse field of view to capture images of the fine-scale structures within the solar corona. It comprises two nested wide-field telescopes with 2K- × 2K-format APS (advanced photo system) CMOS sensors. The light recorded by WISPR’s inner telescope comes from photons scattered either by dust particles or by electrons in the corona.

Lightening the load

It is little surprise that getting up close to the brightest, hottest — and arguably the most volatile — object in our solar system poses a challenge. The teams of both the ESA and NASA missions had to build systems and structures that could withstand extreme heat, energetic particle radiation, and dense dust, yet ensured a lightweight spacecraft.

“New technologies must be proven to be reliable and present little to no risk to the mission,” NRL’s Howard said. “Solar Orbiter was a cost-constrained mission following ESA’s BepiColombo mission to Mercury. All missions today are cost constrained, which means that they are driven by the requirements; nice-to-haves are not allowed.”

Payload limitations differ on the two spacecraft since their orbits are not the same. The PSP needed a lighter payload mass of 50 kg to reach the sun quickly. In fact, the PSP team was delighted when its craft reached a first orbit around the sun just three months after launch, with a perihelion distance of 0.17 AU or about 25.5 million kilometers.

The Solar Orbiter was able to launch with a more generous payload mass of 209 kg, allowing it to carry 10 instruments compared with PSP’s four. The Solar Orbiter’s instrument array includes an EUV imager, a magnetograph, a coronagraph, a heliospheric imager, an x-ray imager, and a spectral imager.

Innovations to beat the heat

The separate orbits of the two craft mean that they endure substantially different heat fluxes. While ESA’s Solar Orbiter’s perihelion distance of 0.28 AU sees it basking in 12.75× the temperatures felt on Earth’s surface, NASA’s PSP positively swelters in 475× that range.

ESA was able to launch its Solar Orbiter with a payload mass of 209 kg, allowing it to carry 10 instruments, including an EUV imager, a magnetograph, a coronagraph, a heliospheric imager, an x-ray imager, and a spectral imager. Courtesy of ESA.

The Solar Orbiter uses a multilayer stack of titanium foil sheets, with the outermost layer coated with black calcium phosphate processed from burnt bone charcoal manufactured by Ireland-based ENBIO.

The black coating, commercially named SolarBlack, has a ratio of thermal absorptivity over IR emissivity of about 1.1 and is able to withstand temperatures in excess of 500 °C (773 K). “The coating and foil have mainly been chosen for these two properties: keeping the thermo-optical properties throughout the changing conditions of the orbit, and very low outgassing or particle shedding at high temperatures,” the ESA’s Marirrodriga said.

Due to SolarBlack’s high emissivity and extremely high solar absorbance, the coating can efficiently absorb incoming solar radiation and reemit it to the surrounding cold of space. The rest of the heat shield comprises 18 additional layers of thin titanium finished off with a thicker 25-µm layer.

“The heat shield laterally radiates the absorbed heat, while the radiation and conduction to the main body of the spacecraft is suppressed by the use of thermal blankets and special mounting blades,” Marirrodriga said. “So, in spite of receiving about 110 kW of heating power from the sun at perihelion, the shield limits the heat flux to the main body to less than 15 W.”

The twin tails of comet NEOWISE are seen from the WISPR instrument. The lower tail appears broad and fuzzy, and is created when dust lifts off the surface of the comet’s nucleus and trails behind. Scientists hope to use WISPR’s images to study the size of these dust grains, as well as the rate at which the comet sheds them. The upper tail is the ion tail, which is made up of gases that have been ionized by the sun’s intense light. The PSP’s images appear to show a divide in the ion tail, revealing that NEOWISE may have two ion tails, but more data is needed for confirmation. The image has been processed to increase contrast and remove excess brightness from scattered sunlight to reveal more detail in the comet’s tails. Courtesy of Courtesy of NASA/Johns Hopkins APL/Naval Research Lab.

While Solar Orbiter’s shield was designed to flex in temperatures that range from 165 K in planetary eclipse to 293 K at launch and up to 755 K in flight, the PSP opted for a rigid design. Using carbon-carbon layers on the top and bottom of a 6-in.-thick carbon foam, the PSP’s heat shield weighs just 73 kg, yet withstands temperatures reaching to 1643 K.

The sun-facing side of the heat shield is also sprayed with an alumina ceramic-like white coating to reflect as much of the sun’s energy away from the spacecraft as possible.

Both shields are so effective that the subsystems in their shadow operate in temperatures similar to those felt on Earth. This means that any system under the protective shadow of either heat shield can utilize existing technologies. Not so, however, for any equipment lying outside of the shadow.

“The solar arrays needed to be able to handle the heat, and the two programs had different requirements and different solutions,” Howard said. “Orbiter, which goes to 0.28 AU, added a rotation to the arrays to reduce the input. PSP feathers the arrays so that only a small area remains in sunlight, but also added a water-cooling system. All three of these modifications had not been used before.”

Future orbit

Marirrodriga believes that the main challenges that lie ahead now are of a scientific nature, and he is looking forward to analyzing the data. Interpreting the results of these missions will bring about many discoveries and lead to a better understanding of the sun, the driving forces of the solar wind, and the dynamics in the magnetic fields that can explain the 11-year solar cycle.

“These, in turn, will help humankind to travel farther into the solar system and understand the impact of solar activity on our lives on Earth,” he said.

The Solar Orbiter’s first light on May 15 captured visible light images showing the two bright equatorial streamers and fainter polar regions that are characteristic of the solar corona during times of minimal magnetic activity. Courtesy of ESA and NASA.

For NRL’s Howard, it has become obvious that the only way to continually image phenomena on the sun — at all wavelengths from IR, through the visible, UV, EUV, x-ray, to gamma rays — is to do so from space.

“These events can be tracked on their way to Earth. The weather satellites looking at Earth providing amazing images of weather patterns will be joined by those imaging the ionosphere and magnetosphere, giving us warnings of space weather outages,” he said. “I am convinced that there will soon be an armada of solar observing instruments around the sun.”